INTRODUCTION :
============
Encrypting files is very useful and important nowadays, but many OSs do not
support this feature natively(yet). This project is an attempt to create a 
new system call that can take an input file, encrypt or decrypt it, and then
produce an output file.

 
DESIGN APPROACHES :
=================
1. SELECTING THE KEY
    - In this implementation, a fixed length (16 bytes) key is used 
      for encryption and decryption.
    - This key is selected from keybuf which is SHA1(user_password)
    - First 16 bytes for this 20 bytes hash is used as the key for
      [en|de]cryption

	-------------------                     --------
	[en|de]cryption key = First 16 bytes of  keybuf
	-------------------                     --------
	    16 bytes				20 bytes

2. HANDLING FILES THAT ARE NOT MULTIPLE OF CIPHER BLOCK ENCODING SIZE
    - To handle these cases, AES in CTR mode 
    - This mode does not require any special measures to handle messages 
      whose lengths are not multiples of the block size, since this mode
      works by XORing the plaintext with the output of the block cipher

3. PREAMBLE DETAILS 
    - The preamble of the encrypted file is of length : 32 bytes
    - These 32 bytes can be divided into two segment, each of 16 bytes
	- First 16 bytes :
	    - This segment contains a hash value (say 'H').
	    - This is calculated as, H = MD5(keybuf) 
		where, keybuf = SHA1(user_password)
	    - The main reason why the SHA1 hash is rehashed using MD5 
	      is to ensure that the [en|de]cryption key is not publicly 
	      available. 		
	 
	- Last 16 bytes :
	    - This segment holds the initialization vector (IV).
	    - This IV is used by AES.

	--------   -------------------   ---------------------
	preamble = md5(sha1(password)) + initialization vector
	--------   -------------------   ---------------------
	32 bytes        16 bytes	       16 bytes

4. IV CALCULATION
    - The IV is calculated using a combination of inode number of temp file
      and the page number. 
    - The inode of the temp file is used here, as opposed to the inode num
      of infile or outfile, as it adds a level of randomness to IV generation.
    - This means that for the same file that is encrypted, the IV is not
      necessarily the same.
    - This IV so generated is saved in the preamble. These bits are read 
      at the time of decryption to ensure same IV for [en|de]cryption.
    - This additionally ensures that no two blocks for a given file have 
      the same IV, making the encryption more secure from attacks.
    - Also from a security stance, it is always suggested to have diffent IVs, 
      even if the passwords are same.
		 
	---------------------   -----------   ------------	
	initialization vector = page number + inode number
	---------------------   -----------   ------------
	       16 bytes           8 bytes       8 bytes	


5. RESTORING PREVIOUS STATE IN CASE OF FAILURE
    - To ensure that the state of the fs is maintained in case of failure 
      (during read-write, [en|de]cryption etc.), a temporary file is used
    - All the writing is done to this temporary file. On success, this file
      is renamed to the outfile name given by the user. Alternatively, on
      failure the previous state of the fs is restored by unlinking the 
      the temp file.      

6. OTHER DETAILS
    - Made use of locking for vfs_rename, to handle cases when multiple 
      threads operate on a single file.
    - The naming of the temp file is <outfile_name>.tmp .This ensures that 
      the tmp files are all unique and no inconsistencies arise.
    - Made use of a macro to check the max length of the file name.
    - Made use of simple arithmetic to make sure block level read-write 
      occures. This ensures that the kernel load is independent of the file
      size. 


FUNCTIONAL DESCRIPTION :
======================
There are 6 utility functions, each one defined to handle a specific task 
(mentioned below) inorder to achieve encrytion, decryption and copying 
based on the input flags.
						
	--------------------------------------------------------------
	  Task					: Function Name
	--------------------------------------------------------------
	- Input Validation			: cpenc()
	- Read-Write Operatio			: file_read_write()
	- Preamble Handling			: preamble_handler()
	- IV Initialisation			: init_iv()
	- Optional Encryption-Decryption	: encrypt_decrypt()
	- Renaming				: rename()
	- MD5 evaluation			: get_md5_digest()
	--------------------------------------------------------------


AUTHOR
======
Anirudh Kulkarni